Re: [PATCH 2/2] KVM: x86: optimize delivery of TSC deadline timer interrupt
On Sat, Feb 07, 2015 at 09:33:09PM +0100, Paolo Bonzini wrote: Can remove another 300 cycles from do_div when programming LAPIC tscdeadline timer. Do you mean using something like lib/reciprocal_div.c? Yes. Good idea, though that's not latency, it's just being slow. :) It adds to latency, yes (the guest will reprogram the APIC timer before resuming userspace execution). -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH 2/2] KVM: x86: optimize delivery of TSC deadline timer interrupt
On 06/02/2015 21:51, Marcelo Tosatti wrote:
On Fri, Feb 06, 2015 at 01:16:59PM +0100, Paolo Bonzini wrote:
The newly-added tracepoint shows the following results on
the tscdeadline_latency test:
qemu-kvm-8387 [002] 6425.558974: kvm_vcpu_wakeup: poll time
10407 ns
qemu-kvm-8387 [002] 6425.558984: kvm_vcpu_wakeup: poll time 0
ns
qemu-kvm-8387 [002] 6425.561242: kvm_vcpu_wakeup: poll time
10477 ns
qemu-kvm-8387 [002] 6425.561251: kvm_vcpu_wakeup: poll time 0
ns
and so on. This is because we need to go through kvm_vcpu_block again
after the timer IRQ is injected. Avoid it by polling once before
entering kvm_vcpu_block.
On my machine (Xeon E5 Sandy Bridge) this removes about 500 cycles (7%)
from the latency of the TSC deadline timer.
Signed-off-by: Paolo Bonzini pbonz...@redhat.com
---
arch/x86/kvm/x86.c | 14 +-
1 file changed, 9 insertions(+), 5 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0b8dd13676ef..1e766033ebff 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6389,11 +6389,15 @@ static inline int __vcpu_run(struct kvm *kvm, struct
kvm_vcpu *vcpu)
!vcpu-arch.apf.halted)
return vcpu_enter_guest(vcpu);
-srcu_read_unlock(kvm-srcu, vcpu-srcu_idx);
-kvm_vcpu_block(vcpu);
-vcpu-srcu_idx = srcu_read_lock(kvm-srcu);
-if (!kvm_check_request(KVM_REQ_UNHALT, vcpu))
-return 1;
+if (kvm_arch_vcpu_runnable(vcpu))
+clear_bit(KVM_REQ_UNHALT, vcpu-requests);
+else {
Why the clear_bit? Since only kvm_vcpu_block in the below section
sets it, and that section clears it as well.
You're right.
Can remove another 300 cycles from do_div when programming LAPIC
tscdeadline timer.
Do you mean using something like lib/reciprocal_div.c? Good idea,
though that's not latency, it's just being slow. :)
Paolo
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Re: [PATCH 2/2] KVM: x86: optimize delivery of TSC deadline timer interrupt
On Fri, Feb 06, 2015 at 01:16:59PM +0100, Paolo Bonzini wrote:
The newly-added tracepoint shows the following results on
the tscdeadline_latency test:
qemu-kvm-8387 [002] 6425.558974: kvm_vcpu_wakeup: poll time
10407 ns
qemu-kvm-8387 [002] 6425.558984: kvm_vcpu_wakeup: poll time 0
ns
qemu-kvm-8387 [002] 6425.561242: kvm_vcpu_wakeup: poll time
10477 ns
qemu-kvm-8387 [002] 6425.561251: kvm_vcpu_wakeup: poll time 0
ns
and so on. This is because we need to go through kvm_vcpu_block again
after the timer IRQ is injected. Avoid it by polling once before
entering kvm_vcpu_block.
On my machine (Xeon E5 Sandy Bridge) this removes about 500 cycles (7%)
from the latency of the TSC deadline timer.
Signed-off-by: Paolo Bonzini pbonz...@redhat.com
---
arch/x86/kvm/x86.c | 14 +-
1 file changed, 9 insertions(+), 5 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0b8dd13676ef..1e766033ebff 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6389,11 +6389,15 @@ static inline int __vcpu_run(struct kvm *kvm, struct
kvm_vcpu *vcpu)
!vcpu-arch.apf.halted)
return vcpu_enter_guest(vcpu);
- srcu_read_unlock(kvm-srcu, vcpu-srcu_idx);
- kvm_vcpu_block(vcpu);
- vcpu-srcu_idx = srcu_read_lock(kvm-srcu);
- if (!kvm_check_request(KVM_REQ_UNHALT, vcpu))
- return 1;
+ if (kvm_arch_vcpu_runnable(vcpu))
+ clear_bit(KVM_REQ_UNHALT, vcpu-requests);
+ else {
Why the clear_bit? Since only kvm_vcpu_block in the below section
sets it, and that section clears it as well.
Can remove another 300 cycles from do_div when programming LAPIC
tscdeadline timer.
+ srcu_read_unlock(kvm-srcu, vcpu-srcu_idx);
+ kvm_vcpu_block(vcpu);
+ vcpu-srcu_idx = srcu_read_lock(kvm-srcu);
+ if (!kvm_check_request(KVM_REQ_UNHALT, vcpu))
+ return 1;
+ }
kvm_apic_accept_events(vcpu);
switch(vcpu-arch.mp_state) {
--
1.8.3.1
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